Various Flow Control Structures for Writing PLC Programs

In PLC programming, there are various flow control structures used to organize code, implement logical functions, and decision-making processes. When using ST (Structured Text), a programming language defined by the IEC 61131-3 standard, we can utilize control structures similar to those in high-level programming languages. Below are some commonly used flow control structures in ST along with their examples:

Sequential Structure (Sequence)
Sequential structure is the most basic program structure, where statements in the program are executed in the order they appear. In ST, by default, statements are executed sequentially without the need for special syntax to indicate this.

Selection Structure (Selection)
The selection structure allows the program to choose different execution paths based on conditions. In ST, this is typically implemented using IF-THEN-ELSE statements. Example:

IF (Input1 = TRUE) THEN    // If Input1 is true, execute the code here    Output1 := TRUE;ELSIF (Input2 = TRUE) THEN    // If Input1 is false and Input2 is true, execute the code here    Output2 := TRUE;ELSE    // If both Input1 and Input2 are false, execute the code here    Output3 := TRUE;END_IF;

Iteration Structure (Loop)
The iteration structure allows the program to repeatedly execute a block of code until a certain condition is met. In ST, this can be achieved using loop statements such as FOR, WHILE, or REPEAT-UNTIL. Example of a FOR loop:

FOR i := 1 TO 10 DO    // The code here will execute 10 times, with i's value from 1 to 10    Array[i] := i * 2;END_FOR;

Example of a WHILE loop:

WHILE (Counter < 10) DO    // While Counter is less than 10, execute the code here    Counter := Counter + 1;END_WHILE;

Example of a REPEAT-UNTIL loop:

REPEAT    // The code here will repeat until the condition in UNTIL is met    // ... perform some operations ...UNTIL (SomeCondition = TRUE);

Nesting Structure (Nesting)
Nesting structure refers to using one control structure within another. For example, you can use another IF statement inside an IF statement, or use a selection structure within a loop, etc. Example (Nested IF):

IF (MainCondition = TRUE) THEN    IF (SubCondition1 = TRUE) THEN        // Execute when both MainCondition and SubCondition1 are true    ELSIF (SubCondition2 = TRUE) THEN        // Execute when MainCondition is true and SubCondition1 is false, but SubCondition2 is true    END_IF;END_IF;

These flow control structures can help you write more structured and understandable PLC programs.